With the increasing availability of high quality and low cost digital imaging devices such as scanners and printers, counterfeiting of printed materials has become an increasingly pervasive problem. In the field of secure printing, a number of optical security features have been developed to combat counterfeiting. These technologies include machine-readable optical features, such as bar codes, holograms, and more recently, digital watermarks. Some of these technologies are not practical for authenticating certain types of printed documents because they involve complex and expensive manufacturing or printing processes.
Digital watermarking is a process for modifying physical or electronic media to embed a hidden machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by subtly altering the host media signal. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting watermarks that are imperceptible to humans in media signals are detailed in the assignee's co-pending application Ser. No. 09/503,881 (Now U.S. Pat. No. 6,614,914) and U.S. Pat. No. 6,122,403, which are hereby incorporated by reference.
Digital watermarks embedded in images may be designed to survive printing and scanning of the images. This type of digital watermark is sometimes referred to as a robust watermark because it survives the distortion of the image signal due to the print and scanning process. The watermarking process starts by embedding the digital watermark in a digital image. The resulting watermarked image is then printed. To read the digital watermark, an image capture device such as scanner or digital camera scans the printed image and generates a digital image. A watermark reader then processes the image to detect the watermark and extract message symbols from it.
Digital watermarks can be used to differentiate between an original and a copy of a printed article. Examples of such techniques are described in PCT Patent Application WO 99/36876. By analyzing attributes of the digital watermark in a suspect item, a watermark reader can differentiate between a copy and an original. For example, changes to the digital watermark, or degradation of it, can differentiate between an original and a copy. Digital watermarks in these types of applications are sometimes referred to as fragile watermarks or semi-fragile watermarks because they are designed to degrade in a predictable way in response to certain types of distortion.
One challenge in using fragile watermarks to authenticate printed articles is determining the nature and extent of the changes of the digital watermark that differentiate originals and copies of a printed item.
The invention provides an authentication system and related methods for authenticating printed objects. The system uses an information-based metric along with one or more print quality metrics to provide accurate detection or classification of a counterfeit printed object. The print quality metric evaluates attributes of a subject image associated with the original printer, ink or paper to detect changes of those attributes due to copying operations like image scanning and halftone printing subsequent to the original printing of the object. The information-based metric measures message symbol errors in an optically readable code, such as a digital watermark.
One aspect of the invention is an authentication system comprising a print quality processor, a message decoder and a classifier. The print quality processor measures one or more print quality metrics from a subject image scanned from a printed object. The message decoder extracts message symbols from the subject image and determines an information metric based on accuracy of the symbols extracted from the subject image. In response to receiving the print quality and information-based metrics, the classifier classifies the subject image as an original or copy as a function of the metrics.
Another aspect of the invention is a method for embedding auxiliary information into an image to be printed on a printed object and used to authenticate the printed object. The method embeds auxiliary information into the image based on a print attribute so that the auxiliary information degrades in response to a copy operation on the printed object. It also embeds print information about the print attribute into the image, such that the auxiliary information includes the print information. The print information includes information about the printer, ink and/or paper that is useful in analyzing an image scanned from the printed object to determine whether the object is a counterfeit.
Yet another aspect of the invention is a printed object authentication system comprising a print quality processor, a digital watermark decoder and a classifier. The print quality processor measures one or more print quality metrics from a subject image scanned from a printed object. The digital watermark decoder detects a digital watermark from the subject image and determines a digital watermark metric based on the detected digital watermark in the subject image. The classifier is in communication with the decoder and processor for receiving the print quality and digital watermark based metrics and classifying the subject image as an original or copy as a function of each of the metrics.
Further features will become apparent with reference to the following detailed description and accompanying drawings.